What is the significance of the statement that 'a man cleaves to his wife and they become one flesh'?

1.

Rashi: The Torah is simply saying that a baby is formed by its father and mother, and it is through him that they become one.

2.

Ramban: The Torah is teaching us that, due to the formation of Chavah from Adam, people are different than animals, inasmuch as whereas a male animal will be intimate with any female it chooses, a man designates one wife, with whom he lives permanently.

3.

Moshav Zekenim : We infer that he clings to Ishto (Nesu'ah), but not when she is betrothed, like Birkas Eirusin "she'Asar Lanu Es ha'Arusos."